The present invention relates to upgrading low value, heavy hydrocarbons, i.e. converting heavy hydrocarbons into more valuable, lower molecular weight (or “lighter”) hydrocarbons, using supercritical water (“SCW”).
Heavy hydrocarbons may be upgraded in partial oxidation processes. “Partial oxidation” refers generally to the combustion of a fuel using a sub-stoichiometric amount of oxygen (“O2”) to produce a “synthesis gas” (or “syngas”) comprising carbon monoxide and hydrogen. The synthesis gas (which also contains methane and carbon dioxide) can then be converted into light hydrocarbons in, for example, a Fisher-Tropsch process.
Broadly speaking, there are two types of partial oxidation process, i.e. thermal partial oxidation (“TPOX”) and catalytic partial oxidation (“CPOX”). Current TPOX processes generally require high temperatures, typically above 1600K, and high pressures, typically from 40 to 70 bar, and efficiency is rather low (e.g. 70% dry gas efficiency for a Texaco/GE quench gasifier and 80% for a Shell dry feed gasifier). Accordingly, there is a need to develop an alternative process to the current TPOX process to improve the efficiency of upgrading heavy, low value, hydrocarbons.
Molecular modeling studies carried out under the direction of the Inventor predicted that partial oxidation of heavy hydrocarbons in SCW with pure oxygen gas would be rapid at lower temperatures, e.g. from 600K to 1000K, and pressures, e.g. 250 bar to 350 bar. Importantly, these studies predicted the same spectrum of products as produced in conventional TPOX reactions. Unexpectedly, experimental studies revealed that, in contrast to the results predicted by the molecular modeling studies, lower molecular weight hydrocarbon compounds are produced directly from reaction of heavy hydrocarbon feedstock in SCW.
SCW is already known to a certain extent for use in processes to convert hydrocarbon compounds. For example, it has been reported (“Pyrolysis of eicosane in supercritical water”; Vostrikov et al; Russian Chemical Bulletin, Int. Ed.; Vol. 50, No. 8, pp. 1478-1480, August 2001) that eicosane can be converted into a mixture of methane, carbon monoxide, carbon dioxide and hydrogen by heating in SCW at 30 MPa (300 bar) and at a temperature from 450° C. to 750° C. (˜723K to ˜1023K). It has also been reported (“Naphthalene oxidation in supercritical water”; Vostrikov et al; Russian Chemical Bulletin, Int. Ed.; Vol. 50, No. 8, pp. 1481-1484, August 2001) that naphthalene can be converted into a mixture of benzene, toluene, methane, hydrogen, soot and carbon oxides by heating in SCW at 30 MPa (300 bar) and at temperature from 660° C. to 750° C. (˜935K to ˜1025K).
U.S. Pat. No. 4,421,631 (Ampaya et al; published in 1983) discloses a process for upgrading a heavy hydrocarbon material, e.g. a petroleum residual, using a molten salt, e.g. alkali metal carbonate(s). Heat for the process is provided by combustion of carbonaceous material, produced as a by product of the upgrading process and entrained in the flow of molten salt, using oxygen.
Processes to upgrade low value, heavy hydrocarbons into more valuable, lighter hydrocarbons using water under supercritical conditions are also known.
U.S. Pat. No. 1,956,603 (White; published in 1934) discloses “aquolysis” processes for converting heavy petroleum hydrocarbons including tars, tarry oils and shale oil into liquids of lower boiling points by heating the heavy hydrocarbons in the form of an emulsion with water at a temperature from 900° F. to 1300° F. (˜755K to ˜980K) and a pressure from below 100 bar to above 1000 bar. It is disclosed that supercritical pressures are preferred.
U.S. Pat. No. 3,989,618 (McCollum et al; published in 1976) discloses a process for upgrading hydrocarbons including heavy materials such as gas oil, residual oils, tar sands oil, oil shale kerogen extracts and liquefied coal products by contacting the hydrocarbons with a water-containing fluid at a temperature in the range of 600° F. to 900° F. (˜590K to ˜755K) and, preferably, at about 705° F. (647K) which is the critical temperature of water. U.S. Pat. No. 3,989,618 exemplifies heating tar sands oil in water (1:3) at a temperature of 752° F. (˜675K) and at a reaction pressure of 4350 psig (˜300 bar) for 3 hours in an autoclave. The tar sands oil was cracked to produce hydrogen, carbon dioxide and methane gases (11.2 wt % total), light ends (75.2 wt %), heavy ends (8.6 wt %) and a solid residue (5.0 wt %). U.S. Pat. No. 3,989,618 also exemplifies semi-continuous flow processing of tar sands oil in water (1:3) at 752° F. (˜675K) and 4100 psig (˜285 bar) in an externally-heated pipe reactor having a reaction volume of about 6 milliliters.
U.S. Pat. No. 4,840,725 (Paspek; published in 1989) discloses a process for converting heavy hydrocarbon oil feedstocks to fuel range liquids. The process involves contacting the heavy hydrocarbons with water (typically, 2:1 to 1:1) at a temperature from 600° F. to 875° F. (˜590K to ˜740K) at a pressure preferably from 4000 psi to 6000 psi (˜275 bar to ˜415 bar). It is disclosed that reaction times are generally short (from a few seconds up to about 6 hours) and that the fuel range liquids produced have increased amounts of high value aromatic carbons. U.S. Pat. No. 4,840,725 exemplifies converting shale oil using a 400 ml vertical tube reactor operating at 825° F. (˜715K) and 4900 psi (˜340 bar). U.S. Pat. No. 4,840,725 acknowledges that coke is produced as a by product of the reaction of feedstock with SCW and indicates that the reaction temperature should not exceed 875° F. (˜740K) in order to minimize formation of this by product.
U.S. Pat. No. 4,818,370 (Gregoli et al; published in 1989) discloses processes for upgrading heavy hydrocarbons using brine under supercritical conditions. It is disclosed that hydrocarbon deposits may be upgraded in situ in subterranean reservoirs and that heat for these processes may be provided by pumping oxygen into the reservoir to combust a portion of the deposits. Temperatures in the combustion zone are allowed to reach ˜478K to ˜1030K at which point the combustion is stopped to allow heat to soak through the reservoir and for the upgrading reactions to occur.
US 2005/0040081 (Takahashi et al; published in 2005) discloses a process for upgrading heavy hydrocarbon oil using SCW at a temperature up to ˜725K to produce lighter hydrocarbons which are combusted in a gas turbine to generate power. Any unreacted hydrocarbon residue is combusted to produce heat which is used, together with heat produced in the gas turbine, to heat water for the process. Further heat for the cracking process is provided externally using a heater. However, it is disclosed that the amount of heat supplied externally may be reduced by reacting a portion of the heavy hydrocarbon with an oxidant.
US 2007/0144941 (Hokari et al; published in June 2007) discloses a process for upgrading heavy hydrocarbon oil using SCW in the presence of an oxidant, e.g. oxygen, to remove vanadium from the heavy oil to ensure that vanadium is not present in the lighter hydrocarbon products.
There is a need for new processes for upgrading heavy hydrocarbon feedstock. New direct conversion processes should be more efficient than existing processes, for example by increasing the overall yield of the lighter hydrocarbons and by improving the spectrum and distribution of hydrocarbons produced. In addition, new processes should improve control of the formation of unwanted solid carbonaceous by-products such as coke and soot.